A CENTURY OF CHEMISTRY. 75 



uranium ore. While demonstrating argon in the 

 nitrogen gas obtained from Cleveite, Prof. Kamsay 

 observed in 1895 another bright yellow line, and this 

 Sir William Crookes recognised as the D line of 

 helium. 



Helium has now been found in many ores, in 

 mineral waters, and in very minute quantities in the 

 air. It is the lightest of all the gases except 

 hydrogen, and Dr. Johnstone Stoney has suggested 

 that this may explain the paucity of these two ele- 

 ments in a free state upon the earth while they are 

 abundant in the universe. As Winkler puts it, " the 

 comparatively small force of the earth's gravitation 

 does not form a sufficient counterpoise to the velocity 

 of their molecules, which therefore escape from the 

 terrestrial atmosphere unless restrained by chemical 

 combination. They then proceed to reunite around 

 great centres of attraction, such as fixed stars, in 

 whose atmospheres these elements exist in large 

 quantities." * 



Helium, like argon, is believed to be monatomic, 

 and it is not known to enter into chemical combina- 

 tion. There remains much uncertainty in regard 

 to its position, some maintaining, for instance, that 

 it is composed of two gases. 



SUMMARY. It is the business of chemistry to 

 distinguish the different kinds of matter, and to 

 study their transformations. Heterogeneous mix- 

 tures have to be distinguished from homogeneous com- 

 pounds and elements. A homogeneous substance 

 which cannot be decomposed by known means is 

 called an element. Careful searching and more ac- 



* Trans, of a paper in Rep. Smithsonian Inst. for 1897, 

 p. 244. 



